The overall goal of these studies is to advance our understanding of the function of a pathogenic T cell response in autoimmunity and to define the structural parameters of the HLA-DR: peptidelig and that stimulates the pathogenic T cell response. Toward this goal, we have made several significant observations regarding the structure and function of the RA associated alleles HLA-DR1 and DR4 in the presentation of auto antigenic peptides. Using a humanized mouse model we have developed for the study of the model auto antigen type IIcollagen (CII), we have identified the CII immunodominant determinants presented by the DR1and DR4 class II molecules, defined the molecular interactions that occur during both the binding of this peptide by these DR molecules and the recognition of this complex by the T cell receptor, solved the crystal structure of HLA-DR1 complexed with the immunodominant CIIpeptide, and developed a means of identifying CII-specific autoimmune T cells ex vivo for highly specific, functional studies of these cells. Based on these data, the focus of this proposal is to investigate the structure-function relationship between the binding affinity of theCII peptide by the DR molecules and the effect of this binding affinity on the stimulation and function of the CII-specific autoimmune T cells. The central hypothesis that we are testing in this proposal is that the low affinity interaction of self peptides with MHC class II molecules induces significant changes in the conformation of the class II: peptide complex, and that this altered structure plays a major role in the stimulation and function of autoimmune T cells. The impact of these studies will be significant to autoimmunity in general in that they address a seminal question - how do MHC molecules mediate susceptibility to autoimmunity? In addition, these studies will identify the phenotype of the autoimmune T cells and the mechanisms by which they function in mediating disease, and will determine how an MHC: peptide low affinity autoimmune response differs from a high affinity foreign anti genresponse. The long term prospect of these studies is that they will provide us with new structural/functional concepts that can be targeted for the development of highly specific immunotherapies for autoimmunity.